The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, the reconstruction of images from data acquired during a helical scan using twin fan beams.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon a linear array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and the linear detector array in a conventional CT system are rotated with a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given gantry angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from a set of data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In order to reduce total scan time when multiple slices are acquired, a so-called "helical" scan is performed where the patient is moved while the gantry revolves to acquire data for the prescribed number of slices. However, helical scanning introduces certain errors with regard to the data in the acquired tomographic projection sets. The mathematics of tomographic reconstruction assumes that the tomographic projection set is acquired along a constant z-axis slice plane. The helical scan path clearly deviates from this condition and this deviation, if uncompensated, results in image artifacts in the reconstructed slice image. The severity of the image artifacts depends generally on the "helix offset" in the projection data, measured as the difference between the table locations of the scanned data and the z axis value of the desired slice plane. Errors resulting from helical scanning will be referred to collectively as "skew" errors.
Several methods have been used to reduce skew errors in helical scanning. A first approach disclosed in copending U.S. Pat. No. 5,046,003 entitled "Method for Reducing Skew Image Artifacts in Helical Projection Imaging" and assigned to the same assignee as the present invention, uses nonuniform table motion to concentrate the helically acquired projections near the slice plane while limiting the accelerative forces on the patient.
In copending U.S. patent application Ser. No. 07/430,372 filed Nov. 2, 1989 entitled "Computerized Tomographic Image Reconstruction Method for Helical Scanning," and assigned to the same assignee as the present invention, skew artifacts are reduced by interpolating between two half scans of data each requiring only 180.degree. plus the fan beam angle of gantry rotation. The half scans require less gantry rotation and hence less table movement, thereby reducing the overall helical offset of the projection data.
In a third approach described in copending U.S. patent application Ser. No. 07/435,980 filed Nov. 13, 1989 entitled "Extrapolative Reconstruction Method for Helical Scanning," and assigned to the same assignee as the present invention, skew artifacts are reduced by interpolating and extrapolating between two partial projection sets of only 180.degree. of gantry rotation. The two partial projection sets require even less gantry rotation than the above half scan approach and, thereby further reduce the overall helical offset of the projection data. Skew artifacts are further reduced by weighting the acquired data as a function of distance from the slice plane as described in U.S. Pat. No. 5,170,346 entitled "Method For Reducing Patient Translation Artifacts In Tomographic Imaging."
These prior methods produce good images when the patient translation speed is moderate, i.e., not greater than 10 mm per gantry rotation. Further reductions in scan time cannot be achieved by increasing patient translation speed because significant image degradations occur.